The art of drilling vertical holes such as oil wells has traditionally utilized a cutting head driven by a linear series of connected pipe lengths, wherein the drilling fluid needed for lubricating and cooling the drill bit passes through the pipe. The weight-on-bit required to cut through the formation is generated from the weight of a drill string. The maximum force which may be generated by such a system is limited by the allowable stresses in the drill string as it acts as a structural column to translate the drilling force to the drill bit.
It is well known in the art of oil drilling that oil deposits may be very difficult to recover through the type of conventional vertical drilling described above due to the tendency of oil deposits to be restricted to narrow "pay" zones which might only be found thousands of feet below the surface. Due to the small diameter of most unmanned oil wells, it is not uncommon for there to be a limited area of exposure of the vertical well to the oil bearing zone. It has been found that the oil recovery rate of these wells can be dramatically increased by forming lateral boreholes which extend from the existing vertical well at an elevation equal to the level of the oil bearing "pay" zone. However, the prior art methods of forming lateral bore holes from within an existing vertical shaft are inadequate for a variety of reasons.
One prior art method of drilling laterally consists of using a flexible drill string such as those shown in U.S. Pat. No. 5,148,875 to Karlsson et al. and U.S. Pat. No. 5,148,877 to MacGregor. These flexible drill strings transfer rotary and compressive forces from the surface to drive the drill bit and cause it to engage the formation being drilled. However, the force these drill strings can apply to the drill bits is limited by the compressive strength of the drill string. Additionally, flexible drill strings typically require a large turning radius when shifting from a vertical to a horizontal direction. This large turning radius makes it difficult, if not impossible, to accurately target the potentially thin oil zone since the drill string must begin its turn at a point well above the target zone. A further problem with flexible drill strings is their tendency to impact the sides of the drilled hole as gravity pulls the string toward the outside of its turn radius. Impacting the side of the hole leads to excessive wear and may cause irreparable damage to the string.
A further method for drilling lateral holes utilizes a self-propelled drilling unit as shown in U.S. Pat. No. 4,365,676 to Boyadjieff et al. The self-contained drilling unit is lowered to a desired level within a vertical hole prior to being activated. The self-propelled unit includes a gripping structure adapted to engage the sidewall of the hole being drilled to thereby transmit the reactive forces of the drilling operation to the sidewall. The unit further contains means for advancing the drill bit relative to the gripping structure to maintain the bit engaged with the formation. However, the self-contained nature of the drilling unit necessarily limits the maximum weight-on-bit which it can generate and may prevent it from developing sufficient force to penetrate hard rock. Furthermore, since the gripping structures only grip the sidewalls of the newly formed lateral borehole, the self-propelled unit may be ineffective in unconsolidated soils since the soil would not provide adequate support to properly brace the drilling unit.
To develop sufficient force for drilling through rock, it is desirable to brace the drill bit against an anchor that will resist the reactive forces developed by the bit, such as the back wall of an existing vertical shaft. This shaft is often lined and thus may better support the reactive drilling forces. One reference showing such a system is U.S. Pat. No. 4,600,061 to Richards which utilizes a manned platform that is lowered into the vertical shaft so that the men thereon may drill the lateral boreholes. However, such a method could not be used with existing vertical oil wells which typically have a diameter on the order of one foot. Furthermore, such a method would place the men on the platform at great personal risk.
It is with regard to this background information that the improvements available from the present invention have evolved.